In “Water III,” filmmaker Morgan Maassen explores the ocean from above and below. I love the sheer variety of fluid phenomena; yes, there are classic breaking barrel waves for surfing, but there are also rib vortices and bubble plumes and churning turbulence that wouldn’t be out of place in a stormy Midwestern sky. Enjoy! (Image and video credit: M. Maassen)
Rip currents — also known as rips — are a threat to beachgoers around the world, and, unfortunately, they’re often underestimated or misunderstood. As waves crash on the shore, water must find a path back out to sea, often through deeper channels that provide a break between the waves. These flow paths are rip currents, and they can form, shift, and intensify with little warning.
Over the years, researchers have found that efforts to educate beachgoers through signs, flags, and other methods once at the beach have done little to help visitors understand, avoid, or escape rips. Instead, it’s better to educate people long before the water is in sight. Since no one method is guaranteed success for escaping a rip, it’s better to learn to recognize and avoid these dangerous areas. Check out the video below for advice on spotting rips, and here’s a video showing rips from a surfer’s perspective, as well as one using dye flow visualization to mark a rip. Be safe and smart out there! (Image credit: P. Auitpol; video credit: Surf Life Saving Australia; via Hakai Magazine; submitted by Kam-Yung Soh)
In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.
Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.
When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)
Surfers flock to northern Peru to enjoy what’s been called the world’s longest wave. These waves are generated by storms thousands of miles away in the Pacific and Southern Oceans. In the open water between, the waves sort themselves into groups of similar wavelength and speed. With the deep water off Peru, the large swells continue to travel together until close to the shore. Surfers also benefit from the tendency for incoming waves to arrive nearly parallel to the coastline, creating long shoreline stretches for breaking. Where many famous wave breaks can be ridden for seconds, surfers can ride these for minutes! (Image credit: L. Dauphin; via NASA Earth Observatory)
In nature, objects like asteroids, black holes, and atomic nuclei can get distorted when spinning rapidly. Researchers are exploring these objects using a new model platform: particle rafts levitated by sound. The individual particles are less than a millimeter wide and tend to clump together due to the scattering of sound waves off neighboring particles. This effect provides a cohesive force — similar to surface tension or the effects of gravity — that draws the particles together. With the right frequency, the sound waves can also make the granular rafts spin, setting up a tug-of-war between cohesion and centrifugal force.
As the rafts spin, they distort, pull apart, and come back together. Interestingly, the cohesive force a raft experiences increases with the raft’s size. That makes the attractive force unlike surface tension (which is the same whether you have a bucket of water or a lake) and more like gravity (which is stronger with more material.) Because of this size dependence, the team hopes their granular rafts could be a new way to study the formation of rubble-pile asteroids and similarly granular systems.
(Video, image, and research credit: M. Lim et al.; via APS Physics)
A word of warning: today’s post includes visuals of digestion taking place in (non-human) embryonic intestines.
Our bodies rely on waves driven by muscle contractions to move both fluids and solids, whether through the esophagus, the ureter, the fallopian tubes, or the intestines. In areas where mixing is unnecessary, those waves move in a single direction, transporting the contents one-way. But in the intestines, mixing is critical to enhancing nutrient absorption, so mammal intestines have wave trains that move both forwards and backwards.
The majority of waves move downstream, carrying waste toward its exit (Images 1 and 2). But occasionally, upstream waves collide with their downstream counterparts to force material together, both mixing and delaying progress in order to allow better nutrient uptake along the intestinal walls (Image 3). (Image credits: top – S. Bughdaryan, others – R. Amedzrovi Agbesi and N. Chavalier; research credit: R. Amedzrovi Agbesi and N. Chavalier; via APS Physics)
As ocean waves crash, they generate aerosols — tiny liquid and solid particulates — that interact with the atmosphere. Curious about the chemistry of these tiny drops, researchers set out to measure their acidity. That’s easier said than done. Over time, aerosol droplets acidify as they interact with acidic gases in the atmosphere and capturing fresh aerosols in the field is next to impossible.
To tackle these challenges, researchers instead moved the aerosols to the laboratory, filling a wave channel with seawater and agitating it to generate aerosols they could then measure. They found that the smallest aerosols become a million times more acidic than the bulk ocean in only two minutes! Find out more about their experiment and its implications over at Physics Today. (Image credit: E. Jepsen; research credit: K. Angle et al.)
How does water drip, drip, dripping onto stones erode a crater? Water is so much more deformable that it seems impossible for it to wear harder materials away, even over thousands of impacts. To investigate this, a team of researchers developed a new measurement technique: high-speed stress microscopy. In the process, they found that water owes its incredible erosive power to three factors: 1) The drop’s impact creates surface shock waves along the material, which helps increase erosive power; 2) After the shock wave passes, a decompression wave in the material helps loosen surface matter; and 3) The spreading drop sends a non-uniform wave of stress across the material that simultaneously presses and scrubs at the surface. Together, these factors enable simple, repetitive droplet impacts to wear away at hard surfaces. (Image credit: cottonbro; research credit: T. Sun et al.; via Cosmos; submitted by Kam-Yung Soh)
When droplets coalesce, they perform a wiggly dance, gyrating as the capillary waves on their surface interfere. When the droplets have matching surface tensions, like the two water droplets in the animation on the lower left, the coalescence dance is symmetric. But for differing droplets, like the water and ethanol droplets merging on the lower right, coalescence is decidedly asymmetric.
The asymmetry arises from the droplets’ different surface tensions. The size and speed of the capillary waves that form on a droplet depend on surface tension, so droplets of different liquids have inherently different capillary waves. During merger, the interference of these capillary waves causes the asymmetry we see. (Image credit: top – enfantnocta, coalescence – M. Hack et al.; research credit: M. Hack et al.)
In 1883, the eruption of Krakatau (also called Krakatoa) shook the world, sending shock waves and tsunamis ricocheting across the globe. Some of the smaller waves hit shorelines in the Atlantic and Pacific that were entire continents and ocean basins away from the original explosion. At the time, scientists were so perplexed by the phenomenon that they blamed coincidental earthquakes for the wave action.
Only when Tonga experienced a similarly devastating volcanic eruption earlier this year were scientists able to verify what they’d long suspected: these smaller tsunamis were not caused by solid material displacing water; instead they are the result of atmospheric pressure waves coupling to the ocean. Follow the full story over at Quanta. (Image credit: M. Barlow; via Quanta; submitted by Kam-Yung Soh)